From the ease of handling thereof, copper powder has been utilized widely in prior art as conducting materials of conductive paste for use in forming conductor circuits by the additive method of screen printing, or for use in various electrical contact members such as for an external electrode of multi layered ceramic capacitors (MLCC).
The above conductive paste can be obtained, for instance, by mixing copper powder with resin such as epoxy resin and various additives such as curing agents thereof, and kneading. The copper powder used in so doing can be fabricated by the wet reduction method (precipitated method), in which deposition is caused by reducing agents from solutions, or the like, containing copper salt, the gas phase reduction method, in which copper salt is thermally gasified and reduced in gas phase, the atomizing method, in which molten copper metal is rapidly cooled with coolant such as inert gas or water to be powderized, and the like.
Among the fabrication methods for copper powder such as those described above, the atomizing method, compared to the generally and widely used wet reduction method, has the advantages of being capable of reducing the residual concentration of impurities in the obtained copper powder, at the same time as allowing less pores to be present in the obtained particle of copper powder throughout from the surface to the interior. Therefore, when used in conducting materials of conductive paste, copper powder fabricated by the atomizing method has the advantages of being capable of reducing the amount of gas generation during paste curing, at the same time as being capable of broadly suppressing the progression of oxidation.
However, while copper powder is suitable in conducting materials of conductive paste owing to high conductivity thereof, as the granularity becomes finer, resistance to oxidation becomes poorer, and in order to improve this, measures have been adopted such as coating the particle surface with silver (Patent Reference 1), which has resistance to oxidation, or coating with an inorganic oxide (Patent Reference 2).    [Patent Reference 1] Japanese Patent Application Laid-open No. H10-152630    [Patent Reference 2] Japanese Patent Application Laid-open No. 2005-129424
Recently, refinement has been sought in forming a circuit with a conductive paste, or the like, and inevitably, refinement has been also sought of the granularity of conducting powder used in conductive paste. Simultaneously, in maintaining stability and reliability of paste properties, variations in shape and granularity have to be small, and conductivity must not be lost. Then, if only an improvement of resistance to oxidation is to be taken, addressing the issue is possible with the technique of Patent Reference 1 or 2, or the like.
However, with the technique of Patent Reference 1 or 2, owing to a dependency on coating techniques, problems arise, not only of requiring large amounts of constituents other than copper that lose conductivity, but also of detachment from the core material copper powder particle. In addition, while it is desirable in reducing the variations in shape and granularity that the constitutive particles are uniformly homogeneous and, furthermore, have low concentration in oxygen content, none that provides satisfaction has been found for such copper powder.
It is an object of the present invention to provide copper powder which, while having fine granularity, does not lose either resistance to oxidation or balance in conductivity, and furthermore, copper powder for conductive paste in which variations in shape and granularity are small and having low concentration in oxygen content.